Introduction Maori, the indigenous population of New Zealand, have the right to good health, healthy conditions and high quality epidemiological data. Consistent, comprehensive ethnicity data are crucial for appropriate representation of Maori health status and in order to monitor governmental progress towards equity in health. Maori have been undercounted in health datasets in the past and different methods of adjusting for this undercount have been developed and used in the calculation of population rates. This study investigates the implications of using four different methods to measure ethnicity in a cohort study.

Methods Using a cohort of patients with ischaemic heart disease (IHD), a sensitivity analysis was applied to determine the impact of four different methods of measuring Maori ethnicity on outcomes that determine disparities in both mortality and receipt of procedures, between Maori and non-Maori with IHD.

Results There was some slight variation in results with the use of different methods to measure ethnicity. Overall however, the interpretation of the results would remain largely unchanged with the use of the different methods. These implications will be discussed. This study has wider considerations for the measurement of ethnicity in a cohort study, the right of indigenous populations to high quality ethnicity data, and the ongoing critique and development of an epidemiology that is responsive to the needs and aspirations of Maori in New Zealand.